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Old 29th September 2003, 02:55 AM   #1
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Default Something strange that happened today...

My 2A3 filaments are fed by current regulated DC. Today I decided to see if 'starving' the filaments made any difference, so I attached a 1 ohm resistor across the filament sockets (2A3 is 2.5V/2.5A, which is 1 ohm), and adjusted the current regulator's pot to have 2.25V across the 1 ohm resistor (that's 10% down from the nominal value).

This had an unexpected side effect. After playing for a few minutes, one of my channels would start making a clicking noise. This eventually happened on both channels, and swapping tubes (driver or output) didn't make a difference. It usually started on a loud transient in the music. Once it started, the only way to make it stop was to shut the amp off. The clicks would appear with a frequency ranging from 1 a second to about 4 a second. Interestingly, turning my scope on or off changed the frequency of the clicks.

The scope showed that this was in the output stage, the clicks weren't showing up on the driver's plate. It was when I hooked up my DMM across the filaments that I noticed something interesting - every time a click occured, the voltage across the filament would drop to 1.5-1.8V, and then climb back up again.

I tried adjusting the current regulator so that the voltage across the filament (measured with the tube in the socket and warmed up for a few minutes) showed 2.5V, but that didn't help. I finally measured the voltage across my regulator's sense resistor (also 1 ohm, with a pot across it with the pot's wiper connected to the ADJ pin), and found that to be really low, about 1.8V. Since I want 2.5A, I need 2.5V across the 1 ohm resistor (which should result in the pot being at its 50% point, since the regulator needs a 1.25V reference). Anyway, I upped the pot so that I had 2.5V across the sense resistor, and that seemed to cure things.

So... what was going on? I guess I really starved my tubes, and they were cutting out? What's strange to me is that I got 2.5V across the tube with a current much lower than 2.5A. And that was my mistake - I was trying to set the regulator based on the voltage across the tube, and that resulted in too low of a current. But, why did that not work? The filament's resistance should start out closer to 0, and climb up to 1 ohm as it gets hot, right? And the current regulator should ensure that it has steady current flow while it's warming up. But I was seeing 2.5V across the filament with less than 2.5A current, which means the filament's resistance was greater than 1 ohm. How did that happen?

Also, is there a 'general consensus' on reducing filament voltage/current (say by 10% or so)? Or is that another of those controversial topics? Does the logic/theory in support of doing this not apply to DHTs, by any chance?

Thanks,
Saurav
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Old 29th September 2003, 04:16 AM   #2
SY is offline SY  United States
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Wild guess: you're causing the regulator to go into limiting or oscillation.
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Old 29th September 2003, 04:36 AM   #3
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Saurav,
did you build the dual regulator current and voltage as you have shown a while ago on another thread? What sequence of regulators did you use? Current-voltage-filament or the other way around?
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Old 29th September 2003, 07:22 AM   #4
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Quote:
Wild guess: you're causing the regulator to go into limiting or oscillation.
I thought about that too (mostly since it's the component I understand the least in there). But what could cause that? It's an LT1085, which is supposed to be good down to pretty low dropout voltage/current levels. And I've added the output cap that the datasheet said was needed "for stability" (though strictly speaking the datasheet had a voltage regulator circuit). Maybe the reduced voltage drop in the filament meant too much power was being dissipated in the regulator, and that caused some kind of thermal shutdown? If that's the case then it's not good, that probably means my heatsink is marginal at best.

Grataku,

I used just one regulator, set up as a current regulator. It's similar to the 2nd schematic in the 'Ronan Reg' diagram I've seen in a few places. I ran out of space in my amp chassis
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Old 2nd October 2003, 10:15 PM   #5
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Default those IC reggy's for heaters... eh......

Re your comments regarding DC solid state in the heater circuit; Although I'm an avid user of switchmode technology for the B+ in tube circuits, (there definite advantages in this)
I'll throw the towel in about using IC reggy's for the heater circuits
as I'm totally against using them. So my tubes get good old AC.
Like all IC regulators, "they can take off" with poor decoupling or long leads to caps where ESR gradually increases and produce noise. If you use electrolytics select 105 type with low ESR.. My motto, if one really has to use these dreaded IC's, use an oscilloscope as well and design for the worstcase.
The cold resistance of heaters will usually force IC regulators to internally current limit until heaters warm up. Essentially IC regulators are very low impedance devices and most have foldback current limiting characteristics (to keep device dissipation low on a s/c). The IC s/c current is way below the cold current of tubes. ...so IC struggles.
For example if you used an IC reggy on an ECL82 heater, it sees the cold resistance of the heater of roughly 1,5 ohms; so, instant switch-on surge is 6.3/1.5= 4.2A. That tube quoted hot (in spec sheets) at 750mA = 8.4 ohms. The situation for larger tubes is far worse.
Those IC blighters have a hard time feeling cold.

AC transformers have excellent surge capabilities to deal with that. However, with AC I suppose everyone has noticed how wonderful the heaters of tube family ECC82/3 types flare up the moment they get juice. ?? soft start will prolong tube life.

r
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Old 2nd October 2003, 10:21 PM   #6
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Well, I'm using the LT1085 as a current regulator, which puts the sense resistor in series with the heater filament. The 2A3 has a 1 ohm filament (2.5V, 2.5A), and I'm using a 1 ohm sense resistor. So even assuming adead short when the filament is cold, the resistance seen by the regulator goes from 1 ohm when cold to 2 ohms when hot. Which isn't great, but it's better than what it would be if it were being used as a voltage regulator.

Your points are well taken though, thank you for the advice.
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Old 2nd October 2003, 11:35 PM   #7
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Hi,

Quote:
However, with AC I suppose everyone has noticed how wonderful the heaters of tube family ECC82/3 types flare up the moment they get juice. ?? soft start will prolong tube life.
You'll only notice heater flare up, as you describe it, with very old ECC series.

More modern tubes/valves designated A have controlled heater warm up.

Either way, no harm's done....the A type should last longer in theory.
I haven't seen any record so far anyway...

Cheers,
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